Carton sealing tapes currently utilize mono- and bi-axially oriented polypropylene as a carrier. The carrier may also be referred to as a tape backing or substrate. Orientation may be achieved through either a double bubble blown process, a mono orienting cast line, or a biaxially orienting cast tenter line. The films can be composed of single or multiple layers. Each layer may consist of a homopolymer, copolymers, or blends of polypropylene.
The disadvantage of the current bi-axially oriented polypropylene (BOPP) films is that they are entirely dependent on polypropylene resins for their construction. As polypropylene (PP) is a by-product of the petroleum cracking industry, its production is dependent upon the production of polyethylene. As production of polyethylene (PE) transitions away from cracking of crude oil (heavies) to refining of natural gas (lights) and its collection by hydraulic fracking, the percentage of PP production decreases per unit of PE production, i.e. decreasing PP supply and increasing its price.
Polyethylene and polypropylene have been utilized in film manufacturing more than any other material. The vast assortment of molecular weights, densities, copolymers available within the respective families make these among the most versatile synthetic materials available for designing plastic articles such as film. PP and PE differ in their intrinsic thermal and mechanical characteristics. This frequently leads to the materials being utilized in order to exploit specific properties lacking in one but present in the other.
Accordingly, there is a need for improved film backings for carton sealing tapes, carton sealing tape itself, and films that include beneficial features and/or performance as a results of the combined use of PP and PE, such as improved tear properties and higher clarity.
Combining PE and PP together, as a blend or as discrete adjacent layers (as disclosed herein), is difficult because of the differences between the polymers. Additional measures are likely to be needed to produce the desired film. One factor making the integration of PE and PP into one film difficult is that PP and PE are not completely miscible.
Another level of complexity is encountered when a film, as proposed herein, contains both PP layers and PE layers. The complexity is especially noticeable when trying to biaxially orient a film that contains these layers. Polypropylene and polyethylene, separately, have been processed in a tenter, machine direction orientation (MDO), “double bubble,” and virtually all these methods result in an oriented finished film. PP and PE, however, have inherent differences in melting temperature, crystallization rates, heat transfer properties, ultimate orientation ratio making simultaneous biaxial orientation thereof very difficult.
Film structures incorporating increasing levels of PE, as disclosed herein, in place of PP will exhibit lower heat resistance, which presents issues for downstream printing equipment that is designed to run PP. Moreover, the stretchability of PE is very limited compared to standard homopolymer PP biaxially-oriented polypropylene grades, which changes the yield stress required (the force required to stretch the film) and the stretch speeds (compare a speed of about 1.5 m/min for PE to about 30 m/min for PP). PE also has a narrower stretching window (due to the significantly lower melting point) that can only vary about 5° C. compared to PP which can vary by about 20° C. The narrow stretching window of PE is a significant limitation for commercial equipment.
Because the MDO process results in extremely large differences in machine direction (MD) and transverse direction (TD) properties, biaxially oriented films are preferred due to their superior balance of MD vs. TD tensile properties. Moreover, machine direction orientation is a component of the more complex biaxial process and occurs either simultaneously with transverse orientation or precedes TD orientation in a sequential (two-stage) tenter process. Biaxial orientation, as just discussed above, adds many levels of complexity to the formation of the films desired and disclosed herein. These difficulties surpass those encountered in films where only monoaxial stretching is utilized. Primarily due to reasons of product quality, the stretching ratios in the MD and TD can only be varied in relatively narrow limits. Stress-induced crystallization during the first orientation sequence necessitates polymer-specific adjustments to orientation temperature to achieve target stretch level. Since orientation equipment generally is designed for a fixed orientation ratio and is relatively inflexible in the transverse direction, hybrid films comprised of polymers having disparate orientation parameters would complicate the optimization of any commercial production of these structures.
Taking all these issues into account, it is no easy or obvious task to formulate a multilayer biaxially-oriented film having coextruded PE/PP layers such as the films disclosed herein.